General Structure and Pathways
On a transverse section of the medulla oblongata, gray matter (forming nuclei) and white matter (representing fiber tracts) are clearly differentiated. Traditionally, these structures are divided into components related to the cerebral cortex (spinocortical loops), the cerebellum (spinocerebellar loops), and other formations.
Functionally and topographically, three main pathway systems are distinguished:
- Dorsal system — contains ascending pathways for deep sensation (proprioception and fine touch).
- Ventral system — contains descending pathways controlling conscious voluntary movement.
- Lateral system — includes tracts connecting the spinal cord to the cerebellum and brainstem centers, regulating coordination and muscle tone.
Decussations: Medial Lemniscus and Pyramids
A hallmark feature of the medulla oblongata is the presence of two major decussations of long fiber tracts.
In the dorsal part, the nucleus gracilis and nucleus cuneatus are located. Sensory pathway axons from the spinal cord terminate here. Fibers originating from these nuclei immediately cross the midline, forming a decussation and a bundle known as the medial lemniscus (lemniscus medialis), which ascends to the thalamus.
In the ventral part, axons descending from the cerebral cortex to spinal motor neurons form two longitudinal bundles called the pyramids. At the border with the spinal cord, a portion of these fibers crosses over (decussatio pyramidum) to the contralateral side as the lateral corticospinal tract, while uncrossed fibers continue descending as the anterior corticospinal tract.
Reticular Formation and Olivary Nucleus
The inferior olivary nucleus (nucleus olivaris) is a large, convoluted structure located between the pyramids and the rubrospinal tract. It receives input from the cerebellum and projects back to the cerebellar cortex and spinal cord, playing a key role in motor coordination and balance.
The central region of the medulla is occupied by the reticular formation. It consists of neurons of varying sizes organized into a network with multidirectional fibers. The reticular formation regulates the baseline excitability of the central nervous system, controls muscle tone via gamma motor neurons, and coordinates stereotyped actions, including respiration.
Vital Autonomic Centers
Damage to the medulla oblongata is fatal due to the presence of critical autonomic centers.
- Respiratory Center: Composed of inspiratory and expiratory neurons whose axons project to motor neurons of the respiratory muscles. The center exhibits intrinsic automaticity and is modulated by blood chemistry, lung stretch receptors (via the vagus nerve), and higher brain centers.
- Cranial Nerve Nuclei: The medulla houses the nuclei of the lower four pairs of cranial nerves (CN IX–XII). Of particular importance are the parasympathetic nuclei of the vagus nerve (CN X), which control heart rate, vascular tone, and bronchial constriction.